Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined. More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
One category of RLG relates to so-called pulsed RLG systems that determine the distance to the surface of the product contained in the tank based on the difference in time (time-of-flight) between transmission of a pulse and reception of its reflection at the surface of the product.
Most pulsed radar level gauge systems employ Time Domain Reflectometry (TDR), which provides a time expansion of the (extremely short) time-of-flight. Such TDR radar level gauge systems generate a transmit pulse train having a first pulse repetition frequency Tx, and a reference pulse train having a second pulse repetition frequency Rx that differs from the transmitted pulse repetition frequency by a known frequency difference Δf. This frequency difference Δf is typically in the range of Hz or tens of Hz.
The transmit pulse train is emitted (non-contact or probe) towards the surface of a product contained in a tank, and the reflected signal is received and sampled with the reference pulse train. At the beginning of a measurement sweep, the transmission signal and the reference signal are synchronized to have the same phase. Due to the frequency difference, the phase difference between the transmission signal and the reference signal will gradually increase during the measurement sweep. This gradually shifting time sampling of the reflected signal will provide a time expanded version of the time-of-flight of the reflected pulses, from which the distance to the surface of the product contained in the tank can be determined.
It is clear that the frequency control of the Tx and Rx signals is critical for the performance of a TDR RLG. Several techniques are currently used to ensure such control.
According to one approach, two matched oscillators (crystals) are used. Two crystals requires a rather long start-up time, in order for the delta frequency to stabilize.
According to a second approach, one single oscillator (crystal) is used to generate both frequencies. One frequency is generated directly from the oscillator frequency or form an integer multiple of that frequency. The second frequency is generated by a gradually increasing phase shift of the first frequency. However, this solution requires components which are susceptible to drift between the two frequencies due to temperature variation and aging.